UNION-EQUAL

(Union-equal x y) equals a list whose members
(see member-equal) contains the members of x and the members
of y. More precisely, the resulting list is the same as one
would get by first deleting the members of y from x, and then
concatenating the result to the front of y.

The guard for union-equal requires both arguments to be true
lists. Essentially, union-equal has the same functionality as
the Common Lisp function union, except that it uses the equal
function to test membership rather than eql. However, we do not
include the function union in ACL2, because the Common Lisp
language does not specify the order of the elements in the list that
it returns.